1. Field of Invention
The present invention relates to an adhesive film including an adhesive layer containing a thermoplastic polyimide on at least one surface of a highly heat-resistant polyimide layer.
2. Description of the Related Art
In recent years, demands for various printed circuit boards have been growing with the development of electronic products each having reduced weight, smaller size, and higher density. In particular, demands for flexible laminates (also referred to as “flexible printed circuit boards (FPCs)”) have been growing. In general, each of the flexible laminates includes a circuit composed of a metal foil on an insulating film.
A typical method for producing a flexible laminate, which is generally composed of various insulating materials, includes bonding a metal foil onto a surface of a substrate, which is a flexible insulating film, using an adhesive material selected from various adhesives by thermocompression bonding. The insulating film is preferably a polyimide film or the like. The adhesive material is typically a thermosetting adhesive, such as an epoxy or acrylic adhesive. Hereinafter, an FPC made using a thermosetting adhesive is also referred to as “three-layer FPC”.
Thermosetting adhesives are advantageous in that they realize bonding at relatively low temperatures. However, since the demands for higher properties, such as heat resistance, flexibility, and electrical reliability, are increasing, the three-layer FPC using the thermosetting adhesive may not be able to meet these stringent requirements. To overcome this difficulty, an FPC that includes a metal layer directly disposed on an insulating film or that uses a thermoplastic polyimide in the bonding layer (hereinafter this FPC is also referred to as “two-layer FPC”) has been proposed. The two-layer FPCs have more desirable properties than the three-layer FPCs and the demands for two-layer FPCs are expected to grow in the future.
Examples of a method for making the flexible metal-clad laminate used in a two-layer FPC include a cast method in which a polyamic acid functioning as a precursor of a polyimide is flow-cast or applied on a metal foil and is then imidized, a metallizing method in which a metal layer is directly formed on a polyimide film by sputtering or plating, and a lamination method in which a polyimide film is bonded to a metal foil with the thermoplastic polyimide. Among these methods, the lamination method is superior to the others in that the range of the thickness of the metal foil usable in this method is wider than that in the cast method and that the equipment cost is lower than that of the metallizing method. Examples of the equipment for the lamination include a hot roll laminator and a double belt press machine that can continuously conduct lamination while unreeling a roll material.
An adhesive film containing a polyimide film having a thermoplastic polyimide layer on at least one surface of the polyimide film has been widely used as a substrate material for the lamination method. Examples of a method for producing the adhesive film containing a polyimide base film include a coating method for coating a thermoplastic polyimide solution or a solution of a precursor of the thermoplastic polyimide onto one or both surfaces of the polyimide base film and drying the solution; a thermal lamination method for bonding a thermoplastic polyimide film onto at least one surface of the base polyimide film under heating; and a coextrusion casting method for casting a polyimide base layer and a thermoplastic polyimide layer at the same time (for example, see Japanese Patent No. 2946416 and Japanese Unexamined Patent Application Publication No. 7-214637). Among these methods, the coextrusion casting method advantageously has productivity and yield higher than those of other methods because the number of steps required is small. However, a conventional coextrusion casting method is required to include a thermal cure step in which imidization is substantially performed by only heating. The thermal cure step disadvantageously has low productivity due to a significantly long time required for imidization and removal by evaporation of a solvent in forming a film. The low productivity leads to an increase in total cost. Thus, by employing the conventional coextrusion casting method, adhesive films are not always provided in cost demanded in the market.